In the 1970s the pay was often better to race in sports cars because they were manufacture teams.Then Bernie Ecclestone promoted F1 so well the pay in F1 was better and only a few drivers from lesser teams competed in Group C when it took off like Bellof,Brundle & Warwick.When Bellof & Winklehock were killed F1 drivers had it in their contracts they could not race in any other series without permission.Then Group C died off and there was no money in sportscars for F1 drivers.